1. Field of the Invention
The invention relates to heating applications and, in particular, to storing thermal energy produced by solar radiation.
2. Description of the Prior Art
Solar radiation appears promising as a potential energy source, and much effort has been expended to developing various machanisms to convert solar radiation to useable power, such as thermal or electrical power. For obvious reasons, energy storage is an important adjunct to any solar radiation power system. Such storage can occur either immediately following collection of solar radiation (thermal storage) or following conversion of solar radiation into, for example, electrical energy (electrical storage).
To be effective and practical, a storage system must be highly reversible. It should be simple, self-regulating, service-free and capable of extended cycle life, high efficiency and rapid charge-discharge cycles.
Thermal energy can be stored by the heating, melting or vaporizing of a material, and the energy becomes available as heat when the process is reversed. For example, the heat capacity of a substance (sensible heat) may be utilized; common examples of such substances include water and crushed rock. Such an arrangement, however, requires extensive insulation in order to retain heat until the period of non-illumination, when it will be needed. Another example of a thermal energy system employs the latent heat evolved upon transition of a substance from one phase to another (e.g. solid to liquid state or liquid to vapor state) at a constant temperature. Such a phase change storage system requires, for example, low melting eutectic materials having high latent heats. However, as heat is withdrawn, the material fuses onto cold surfaces and heat must then be conducted through frozen material. Thus, in addition to the insulation requirement noted above, a material of high thermal conductivity is desirable. As a consequence of the requirements of high latent heat, high thermal conductivity and extensive insulation, economic considerations play an important role in the selection of a suitable material.
Storage systems employing hydrogen-containing compositions have been previously described. For example, in U.S. Pat. No. 3,070,703, issued Dec. 25, 1962 to W. H. Podolny, a storage system is disclosed which includes an electrolysis unit wherein electricity generated from solar energy forms hydrogen and oxygen from water. The gases are stored for later use. Subsequently, recombination of the two gases in a fuel cell to form water produces electricity.
Another storage system is disclosed in U.S. Pat. No. 3,459,953, issued Aug. 5, 1969 to W. L. Hughes et al. There, the storage system includes an electrolysis unit wherein surplus electrical energy generated from solar radiation is utilized to form hydrogen and oxygen from water. The two gases are later reacted in an aphodid burner to produce steam, which is then used to drive a turbine which in turn drives a generator. These systems, however, deal with electrical storage and consequently are not applicable to thermal storage systems.
Hydrogen-containing compositions in thermal storage systems are disclosed in U.S. Pat. No. 1,002,768, issued Sept. 5, 1911 to F. Shuman, which discloses a reversible fluid cycle employing, for example, ammonia dissolved in water. A source of heat (solar radiation) releases the gas. When the gas is later permitted to redissolve in the liquid, heat is evolved by the heat of reaction. A similar system is disclosed in U.S. Pat. No. 1,171,305, issued Feb. 8, 1916 to C. H. Beadle.
Use of metal alkyl hydrides for storing thermal energy is disclosed in U.S. Pat. No. 3,064,640, issued Nov. 2, 1962 to E. E. Donath. That patent, however, is directed to external heating devices which employ an irreversible reaction of the metal alkyl hydride plus air to produce heat.
Metal alloys which form metal hydrides by absorbing hydrogen have been disclosed in, for example, U.S. Pat. Nos. 3,315,479, 3,375,676, 3,508,414 and 3,516,263, issued to R. H. Wiswall, Jr. et al. These patents, however, are directed to storing hydrogen for subsequent use in fuel cells and as rocket fuel. The particular metal alloy hydrides employed feature low heats of reaction in order to reduce heat dissipation problems.
The use of a metal hydride system as a heat pump to generate intermittent high energy power from a low energy isotope source has been disclosed by W. E. Winsche in U.S. Pat. No. 3,504,494. That patent, however, does not encompass the concept of thermal energy storage.
There remains a need for a thermal storage system employing materials having ease of reversibility, high heat of reaction, and ease of control over the rate of heat evolution.